1. Field of the Invention
The invention relates to a device for inscribing and/or reading a magneto-optical information carrier, comprising a frame with a turntable which is rotatable about an axis of rotation for supporting the information carrier, a slide comprising an optical unit, which includes an objective for concentrating a radiation beam to form at least one radiation spot in a focusing plane, rectilinear guide means for the slide, arranged on the frame, a slide drive unit, and a movable magnetic unit which, viewed along said axis of rotation, is arranged opposite to and spaced from the optical unit for generating a magnetic field which extends into said focusing plane.
2. Description of the Related Art
Such a device is known from German Offenlegungsschrift DE 37 23 134 (herewith incorporated by reference). Such prior-art device for magneto-optically inscribing a magneto-optical disc comprises a housing having a cover which can be opened and closed to insert or remove a magneto-optical disc. During operation the magneto-optical disc loaded into the housing is held on a turntable by a disc-pressure member and is rotated by a drive motor. The drive motor is secured to a frame accommodated in the housing. The housing further accommodates a slide which is movable over rectilinear guide means of the frame in a radial direction relative to the axis of rotation of the turntable. The slide is constructed as a tilted U-shaped part having two mutually parallel radial limbs. One of the limbs carries an objective lens for concentrating a radiation beam to form a radiation spot and the other limb carries a permanent magnet for producing a magnetic bias field. The objective lens and the magnet are arranged opposite one another and are each situated at one side of the magneto-optical disc which is supported by the turntable.
An information carrier to be used for magneto-optical recording is provided with a thin film of a ferromagnetic or a ferrimagnetic material having a direction of easy magnetisation perpendicular to the surface of the thin film. During recording it is necessary that the thin film be heated to a temperature above the Curie temperature, or in the case of ferri-magnetic materials to a temperature above the so-called compensation point. Recording can effected in accordance with two methods. In accordance with a first method, for which the known device can be used, a constant magnetic bias field is applied by means of a permanent magnet and the thin film is heated by means of a pulsating laser beam. In accordance with a second method the thin film is locally heated by means of a continuous-wave or pulsed laser beam and a pulsating magnetic field is applied at the location of the thin film heated by the laser beam. The pulsating magnetic field is generated by means of an energised coil. The information recorded by means of either method is read optically, utilising the Kerr effect.
The known device cannot readily be adapted to the second of these methods of magneto-optical recording. In principle, it is conceivable to replace the permanent magnet by a coil, but if it is required to record a large amount of information per unit of time the magnetic flux in the coil has to be minimised. A consequence of this is that the magnetic field which is produced is comparatively small, so that the coil should be arranged at a very short distance from the information carrier. Such a spacing is possible only if the coil is incorporated in an actuator for moving the coil in a direction parallel to the axis of rotation of the turntable. It is found that the U-shaped construction of the slide in the known device causes undesirable dynamic problems during energisation of the actuator. The comparatively compliant construction of the known slide can be stiffened by using a heavier construction for the slide; but this has the drawback that the mass of the slide becomes impermissibly large, and is incompatible with fast access times.